1. Field of the Invention
The invention generally relates to the field of multiple printing or reproduction apparatus and is more particularly concerned with reproduction apparatus in which an electroscopic toner powder image is heated to an extent sufficient to fuse the powder image onto a web or sheet of a carrier material as it passes through a nip formed between a conductively heated fusing roll and a cooperating back-up pressure roll.
2. Description of the Prior Art
In one form of prior art apparatus for fusing toner powder images to a carrier medium, the medium is fed, after the formation of the toner image, through a separate fuser unit before it passes through cooperating exit rollers. The fuser consists of a parallel array of resistance heater wires supported directly from the copier frame in a heat radiating plane just above the plane of the powder image. Arrangements of this kind are discussed, for example, in the R. F. Pichierri U.S. Pat. No. 3,834,805, issued Sept. 10, 1974 for a "Xerographic Copier with Asynchronous Copy Feed" and assigned to Sperry Rand Corporation.
While the Pichierri apparatus has enjoyed successful use and affords a conveniently low warm-up time for the reproducer, the heater wires, to be relatively efficient, are located close to the plane of the paper or other carrier medium. In the event the paper jams, there is a serious possibility of contact between a hot fuser wire and the paper, even though the hot wires are partially shielded, and fire may result or, at least, there will be damage to the medium.
In a second form of prior apparatus for fusing toner powder images to a carrier medium, the medium is fed, after the formation of the toner image, through cooperating rolls, at least one of the rolls being heated by a non-rotating infrared radiator contained in a cylindrical cavity within the heated roll along its axis of rotation. The radiating device is normally a high power lineal quartz heat lamp or tungsten filament lamp, relatively short-lived and relatively inefficient because of its high operating temperature. Due, among other factors, to its non-rotating axial disposition, transfer of heat to the active cylindrical surface of the fusing roll is entirely by radiation and is therefore inefficient and power consumption is large. Such fuser rolls also have extremely large thermal masses and require an undesirably long warm-up time after operating power is first turned on. Thus, most electrographic plain paper copiers must be left on continuously, even if use is intermittant, with consequent added costs. Heat loss from the fusing roller to other parts of the apparatus is a further serious disadvantage of such prior art roll fuser systems.